Many optical instruments are provided with automatic apertures or diaphragms that respond to a light sensor. The automatic diaphragms open up in dim light and close down in very bright light. For example, cameras with f1.2 lenses may close down to f16 in the presence of very bright light and under lesser light open up to various intermediate stops, depending upon the available light, and will be wide open at f1.2 in the dimmest light. Many times, however, the difference in illumination is so great that the automatic aperture cannot respond sufficiently and the camera must then be adjusted, automatically or manually, to a shutter speed that will accommodate the excessively bright scene or the excessively dim scene.
In optical instruments having images viewed by humans, there is no possibility of changing the shutter speed to accommodate the range of illumination that exceeds the diaphragm openings of the lens. If a human is observing an image in dim light, the iris of each eye is at maximum opening. If now the image is suddenly brightly illuminated, each iris will try to reduce its opening, but with a definite time lag on the order of many seconds. The inability of the iris to close rapidly overloads the retina of the eye, causing a blanking out of all images. This momentary blindness is commonly encountered also when a person inadvertently looks at the sun. The combination of slow iris accommodation and momentary blindness in the presence of momentarily bright images incapacitates the human observer, and he is unable to see the scene or follow any action for many seconds.
The inability of a human observer to view a scene that momentarily becomes intensely bright is aggravated when a momentarily bright scene quickly becomes relatively dark again. The iris of each eye previously attempting to close down must now try to open up the aperture to admit more light. Again, there is a time lag. This well known theater effect may take up to two minutes for the eye to accommodate to the darkness. Again, there is a loss of acuity because of the dim image on the retina. What little image is transmitted by the retina is overwhelmed by the continuing bright spot blindness from the preceding bright light.
Neutral density filters have been tried to control this inability of human observers to view momentarily bright scenes, but have not proved to be a good solution, and the images are blurred and indistinct under these conditions.